Heat-sensitive copy-sheet



United States Patent C) 2,995,466 HEAT-SENSITIVE COPY-SHEET David P.Sorensen, St. Paul, Minn., assignor to Mlnnesota Mining andManufacturing Company, St. Paul, Minn., a corporation of Delaware FiledAug. 7, 1959, Ser. No. 832,179

9 Claims. (Cl. 117-36) This invention relates to the thermographicreproduction of graphic originals, and has particular reference to novelheat-sensitive copy-sheets for use therein.

An important thermographic reproduction process. extensively employed inreproducing office correspondence and the like, involves placing aheat-sensitive copy-sheet in heat-conductive pressure-contact with adifferentially radiation-absorptive graphic original and subjecting thelatter to brief intense irradiation, e.g. with radiation rich ininfra-red. The heat-pattern produced in the original transfers to thecopy-sheet; there results a corresponding visible change in the latter.A true reproduction of the original is produced directly, without thenecessity of subsequent treatment with solutions or vapors to develop alatent image, or of other processing.

Stable heat-sensitive copy-sheets useful in preparing reproductions ofgraphic originals by thermographic procedures as thus defined havepreviously been described. One popular variety obtains the desiredvisible change by chemical reaction of components including salts orsoaps of various heavy metals.

The present invention likewise provides normally stable heat-sensitivecopy-sheets which are useful in the direct thermographic reproduction ofgraphic originals, solely through the action of radiant energy andwithout the addition of solutions or vapors or other analogousadjuvants, but which distinguish over the prior art in that theimageforming reaction is caused to occur between exclusively organicinterreactant materials, with formation of an organic dye or color-bodycontaining a doubly bonded nitrogen atom as an essential chromophoregroup.

The heat-sensitive copy-paper of this invention ordinarily includes athin paper support web coated on one surface with a thin layer of avisibly heat-sensitive composition. Various binder materials may beemployed where required for maintaining the heat-sensitive reactants inposition. Such binders may be applied in monomeric or sub-polymeric formand then polymerized in situ, but are more conveniently handled assolutions of the polymeric material in volatile solvents which aresubsequently removed. Additional coatings may be pro vided; for examplea white opaque protective surface layer is frequently placed over aheat-sensitive layer on a transparent paper or film support web; Treatedor impregnated paper, polymeric film, or other analogous fibrous ornon-fibrous webs are useful as support or backing members. Theheat-sensitive materials may be applied as a coating or incorporatedwithin the body of the supporting web. These and other variations andmodifications will become apparent on consideration of the followingseveral illustrative but non-limiting working examples, in which allproportions are given in parts by weight unless otherwise specified.

Example I This example provides a normally stable heat-sensitive2,995,466. Patented Aug. 8, 1961 copy-sheet which converts from anolf-white to a greenish black color at image areas under thermographicreproduction procedures through the controlled formation of an oxazinedye.

Three separate smooth mixtures are first prepared by ball-milling.Mixture A consists of 10 parts of gallic acid, 10 parts of polycarbonateresin (Lexan" polycarbonate resin, and parts of dioxan volatile liquidsolvent. Mixture B is prepared from 5 parts of 3-methyl-4-nitrosophenol, 10 parts of polycarbonate resin, and parts of dioxan.Mixture C contains 5 parts of p-N- methylacetylaminobenzene diazoniumfluoborate, 10 parts of polystyrene resin, and 85 parts of acetone.

Mixture C is first applied as a thin uniform coating on flexibletransparent polyester film (3-mil Mylar polyester film) by knife coatingat an orifice of 2 mils, and dried. Equal weights of mixtures A and Bare then blended together and the mixture applied by knife coating at anorifice of 3 mils over the dried first coating. The dried sheet isheat-sensitive and semi-transparent. A further thin coating of a mixtureof 40 parts of titanium dioxide pigment, 10 parts of butadiene-styrene3:1 copolymer (Pliolite 8-7), and 50 parts of toluene, likewise preparedon the ball mill, is preferably next applied over the heat-sensitivecoating and dried, to provide an opaque oft-white background againstwhich the dark colored image areas produced in the thermographiccopyprocess are distinctly visible when viewed through the transparentpolyester film. Color formation occurs sub stantially instantaneously inthe dry copy-sheet on heating to or above the decomposition temperatureof the diazonium fluoborate complex, i.e. to approximately C. On againcooling the sheet, the dye-forming reaction immediately terminates, sothat the initial sharp outline of the reproduced image areas ismaintained and the background areas remain unaltered.

Other pairs of oxazine-dye-forming organic reactants which have likewisegiven good results in the preparation of heat-sensitive copy-sheetsinclude: 2-hydroxy-3- methyl-S-nitrosobenzoic acid with gallic acid orresorcinol; nitrosothymol with gallic acid or 3-hydroxy-2-naphthoic acidor beta-naphthol; and para-nitrosobenzoic acid with gallic acid. In allcases these inter-reactant oxazine dye progenitors are colorless orweakly colored solids which react readily when mixed together in aqueousacetic acid at room temperature or with gentle warming.

Example 2 in this example the several components of the heatsensitivelayer are supplied from a single coating mixture, while stillmaintaining excellent stability even under high ambient humidity storageand use conditions.

Two mixtures are first separately prepared, e.g. by ball milling. Onecontains 10 parts of N-carbethoxy-p-nitrosodiphenylamine, 10 parts ofmethyl methacrylate resin (Lucite 44" resin), and 80 parts of acetone.The second contains 9 parts of gallic acid, one part of(3-chloropropenyl) benzene, 10 grams of the methacrylate resin, and 80parts of acetone. A blend of two parts of the first and three parts ofthe second is smoothly coated on 25 lb. map overlay tracing paper at anorifice of 3 mils, and dried at room temperature. The resulting lightyellowish sheet forms deep blue heated image areas in the therrnographicreproduction process herein described.

Thecolor is due to the formation of an oxazine dye, as in Example 1.

Another dye-forming combination employing the same N-carbalkoxy nitrosocompound and which has provided useful heat-sensitive copy-sheets instructures as here indicated containsN-carbethoxy-p-nitrosodiphenylamine and 3-hydroxy-2-naphthoic acid.

Example. 3

The present example likewisb provides heat-sensitive copy-sheets havingexcellent stability and light background color, and producing dark blueor blue-black heat-image areas.

Three parts of a first mixture of 6.8 parts of N-nitrosodiphenylamine,8.2 parts of ethyl cellulose, and 85 parts of a 55:45 blend of acetoneand heptane, are uniformly blended with two parts of a second mixture of6.8 parts of protocatechuic acid, 1.0 part of alpha-naphthoylchloride,8.2 parts of ethyl cellulose, and 84 parts of the acetoneheptane blend.The composition is coated on map overlay tracing paper using a coatingorifice of 3 mils, and the sheet is dried at room temperature. Theoff-white sheet product converts to intense blue at heated image areasin the thermographic reproduction process.

Equally effective copy-sheets are produced on substitution of equivalentamounts of N-nitrosophenylbenzylamine for the N-nitrosodiphenylamine.

In each of the foregoing examples the principal colorbody produced inthe heated copy-sheet is an oxazine dye; and in each instance colorformation proceeds by a mechanism involving conversion to reactive formof an initially non-reactive progenitor material in the presence of anacidic component. Thus in Example 3 the non-reactive N-nitroso compoundfirst rearranges to a C-nitroso compound; in Example 2 the carbalkoxygroup, which initially prevents coupling by steric hindrance, isdecomposed and removed; and in each of'Examples 1-3 the nitroso compoundas thus formed or as initially present is converted to an intermediatereactive oxime which immediately couples with the aromatic hydroxycompound, producing an oxazine dye wherein a doubly bonded nitrogen atomis an essential chromophore.

A somewhat similar sequence occurs in each of the following examples, inwhich other classes of color-bodies form the image areas.

Example 4 Transparent polyester film is first coated with a two-millayer of a smooth dispersion of 5 parts of para-aminoacetanilidebenzenediazonium fluoborate in a solution of parts of ParapoP'isobutylene-styrene resin in 85 parts of methylcyclohexane. Over thedried coating there is then applied a further 3-mil layer of a blend ofequal parts of a first mixture of 10 parts of 3,3'-dimethoxy-4,4'-diphenylenebis(3-methyl-3-phenyl-l-triazene), 10 parts of Dow 700polystyrene resin, and 80 parts of acetone, and a second mixture of 10parts of Naphtol AS-Supra" azo coupler, 10 parts of polystyrene resin,and 80 parts of acetone. The dried sheet is light yellow in color,stable under all normal ofiice handling and filing, and convertible todeep blue at heat-image areas under thermographic reproductionconditions. The color-body produced is an azo dye, containing doublybonded nitrogen as an essential chromophore.

The two layers may be reversed, the triazene-coupler layer being coateddirectly on the transparent film; or the components of the two layersmay be mixed together and applied simultaneously. In the absence of theacid-liberating component, the triazene-coupler layer of this exampledoes not form a visible image when subjected to thermographicreproduction processes in commercially available equipment. With theacid-progenitor present, cleavage of the normally stable triazene occursunder the thermographic heating, with formation of an intermediate diazocomponent which then immediately couples with the azo coupler component.

-no discoloration or background darkening in normal storage and use.

Typical tn'azenes which have been used in producing these stablecopy-sheets include l,3-bis(2-carboxyphenyl) triazenc,l-(2-car-boxyphenyl)-3-phenyl-3-ethyltriazene, l-(2-methyl-4-chlorophenyl) 3 (p N methylacetamidophenyl) triazene,N-phenylazoisatoic anhydride, 3,3'-dimethoxy 4,4 diphenylenebis( 3methyl-B-phenyl-l-triazene), 3phenyl-4-oxo-3,4-dihydro-l,2,3-benz.otriazine, and4-hydroxy-1,2,3-naphthotriazine.

Other somewhat more reactive triazenes are known which in dry sheet formpossess adequate stability and undergo a color-producing reaction withthe azo coupler component at or near maximum commercially availablethermographic copying conditions even in the absence ofacid-progenitors. The incorporation of the diazonium fiuoborate ofExample 4 or analogous sources of Lewis acids in such sheets greatlyincreases the rate and decreases the temperature required for producingelfective copy of graphic originals on copy-sheets employing suchcompounds, while having no deleterious effect on the stability of thesheet. Typical of such triazenes are 1-(2-carbomethoxyphenyl)-3-phenyltriazene andl-(p-N-methylacetamidophenyl)-3-phenyltriazene. Compounds such as3,3'-dimethoxy-4,4'-diphenylenebis(3-methyl-3 (sodium methylenecarboxylate)-1-triazene) and 3,3'-dirnethoxy-4,4'-diphenylenebis(3-methyl-3-(sodiumbetaethylenesulfonate)-l-triazene) are likewise useful and, while notstrictly organic, on cleavage yield reactive organic diam intermediatesand are considered to be organic triazenes.

Although aromatic amine compounds and active methylene compounds knownto be useful as azo coupling components are also applicable in thepreparation of these copy-sheet materials in many instances, it ispreferred to employ the more strongly reactive aromatic hydroxy azocouplers in copy-sheets prepared as in Example 4; and of these, the arylamides of 3-hydroxy-2-naphthoic acid, known commercially as the NaphtolAS series of azo couplers, are found to be particularly desirable.Exemplary compounds are 3-hydroxy-2-naphthanilide and 3-hydroxy-N-Z-naphthyl-Z-naphthamide.

- In addition to those illustrated in Examples 1 and 4, other diazoniumfluoborates useful in these heat-sensitive copy-sheet formulationsinclude para-acetylaminobenzenediazonium fluoborate, metaandortho-carboxybenzenediamnium fluoborate, orthophenylbenzenediazoniumfluoborate, 3,5-dimethylbenzenediazonium fluoborate, para-(bisbenzenediazonium fluoborate), and ortho-carboxybenzene-diazoniumfiuoborate. These compounds may effectively be replaced by analogousbenzenediazonium hexafluorophosphates, exemplified by such compounds aspara-anilinobenzenediazonium hexafluorophosphate, para- N.-methylacetylaminobenzenediazonium hexafluorophosphate,para-acetylaminobenzenediazonium hexafluorophosphate, andpara-chlorobenzenediazonium hexafluorophosphate. Each of these compoundsis decomposable on heating under the time-temperature conditions of thethermographic reproduction process, with liberation of a Lewis acidwhich will be recognized from the above as including BF, and PF Example5 Still a further specific illustration is provided by applying to atransparent thin carrier web a 3-mil coating of a mixture of 1 part ofalpha-chlorophenylacetate, 5 parts of para-aminoacetanilide, 10 parts ofpolyvinyl formal (Formvar) resin, and 84 parts of ethanol. Over thedried coating is next applied a thin opacifying layer of 40 parts oftitanium dioxide, 10 parts of Pliolite S-7 hutadienestyrene copolymer,and 50 parts of toluene.

The dry sheet is pure white and forms a brown heatimage. The reactioninvolves the formation in the dry heat-sensitive layer of a Schifis basecolor-body by acidinduced liberation of aldehyde from the polyvinylformal followed immediately by inter-reaction of the aldehyde and thep-aminoacetanilide. The color-body contains doubly bonded nitrogen as anessential chromophore. The polyvinyl formal resin serves both as areactant progenitor and as a polymeric binder thus simplifying thepreparation of the coating composition.

The same effect is attained with the same binder resin in conjunctionwith other reactive amino compounds includingp-amino-N-methylacetanilide, dodecylamine, and p-phenylenediamine. Otheracetal-type binders, e.g. polyvinyl butyral (Butvar) resin, are equallyas eflective as the Formvar resin. Inert binders may be included ifdesired, although simplified formulations as described in the exampleare much preferred. Likewise, other equally reactive and otherwisesuitable non-polymeric solid acetals may be incorporated together withsuitable organic amines in a coating composition including an inertbinder component, to provide useful but less desirable heat-sensitivecopy-sheet materials.

The nature of the thermographic reproduction process hereinbeforereferred to makes difficult the direct measurement of the temperaturesattained in the heat-sensitive copy-sheet during said process. Anindirect method of temperature determination is therefore ordinarilyemployed, in which a segment of the copy-sheet is momentarily pressedagainst a heated metal test bar at a known temperature and any visibileeffect on the sheet is noted. Copy-sheets which under such test areconverted from colorless or weakly colored to visibly distinct andrelatively intensely colored form at temperatures within the range ofabout 90-150" C. are found to produce excellent results in terms of thethermographic reproduction of typewritten or analogous graphic originalson th'ermographic copy-machines such as the Thermo-Fax" brand Secretarycopying machines. Copy-sheets prepared in accordance with each of theexamples hereof have been found to be operable within the temperaturerange indicated and to produce usefully clear and distinct thermographiccopies of graphic originals.

The invention is further illustrated in the drawing in which isschematically portrayed the thermographic reproduction of a graphicoriginal 10, having radiationabsorptive inked image areas lll, on acopy-sheet 12 prepared in accordance with the invention and including aheat-sensitive layer 13 comprising inter-reactant components, which onreaction form a color-body containing a doubly bonded nitrogen atom, andan acid progenitor which liberates a Lewis acid on momentary heating.The process involves brief intense irradiation of the composite from asource 14, and results in formation of visibly distinct converted imageareas 15 in the heat-sensitive layer 13.

It will be understood that other equivalent specific structuresemploying the various reactant systems or equivalents thereof in othercombinations are also to be considered as coming within the scope of theinvention. For example, the opaque protective surface coating ofExamples 1 and 5 may be omitted, or may be added to the copy-sheets ofExamples 24 where the carrier web is transparent. The two coatingsforming the heat-sensitive layer of Example 1 may be combined in asingle coating, or separately applied in inverse order. Given a binderof sufiicient strength and in sufficient amount, the heat-sensitivelayer may be made in the form of a self-sustaining film; or the reactantmaterials may be supported within a fibrous web in the virtual absenceof a film-forming binder.

Selection of the specific compound or class of compounds to beintroduced as the acid-progenitor in any particular formulation willdepend not only on the temfit perature at which the color-formingreaction is desired to occur but also on the other specific componentsof the formulation. Thus the fluoborate and fiuophosphate compounds arepreferably employed with binders soluble in hydrocarbon solvents, sincetheir activity is reduced when combined in alcohol with binders solubletherein, presumably by some side-reaction. Again, formulationscontaining reactive amino components which are capable of formingcolor-bodies with diazonium radicals are preferably prepared withactive-halogen-containing acid-progenitors rather than with thefluoborates or the like. On the other hand, since the acid-progenitorscontaining active halogens require the presence of amines, hydroxycompounds, or other sources of active hydrogen atoms for the effectiveliberation of a Lewis acid, other classes of acid-progenitors must beselected for formulations devoid of such compounds or sources.

The active-halogen-containing acid-progenitors of Examples 2, 3 and 5and other useful compounds of the same general structure having one ormore halogen atoms in activated positions on the skeletal carbon chainor ring, may be selected and characterized as being capable ofprecipitating the corresponding silver halide from an alcoholic aqueoussolution of silver nitrate. As an illustration, the addition of asolution of one part of alphanaphthoylchloride in ten parts of ethanolto a solution of one part of silver nitrate in ten parts of ethanol andten parts of water produces an immediate copious precipitation of silverchloride.

What is claimed is as follows:

1. A heat-sensitive copy-sheet adapted for making a clear and sharpreproduction of a graphic original by a dry thermographic processinvolving brief application of a heat-pattern corresponding to saidoriginal, said copysheet being visibly stable under normal storageconditions and being rapidly permanently visibly changed on heating to aconversion temperature within the approximate range of l50 C., saidcopy-sheet including a visibly heat-sensitive layer containing, inintimate association, 1) weakly colored, normally solid, essentiallynon-hygroscopic and non-volatile, inter-reactant progenitors ofintensely colored organic color-bodies containing doubly bonded nitrogenas an essential chromophore,

one of said progenitors being acid-convertible to a modification whichunder acidic conditions is rapidly visibly irreversibly inter-reactivewith another of said progenitors, and (2) acid-progenitor means, stableunder said normal storage conditions and liberating a Lewis acid onbeing momentarily heated at said conversion temperature.

2. A heat-sensitive copy-sheet as defined in claim 1 in which theacid-progenitor means is a Lewis acid complex of a substituted-benezenediazonium radical.

3. A heat-sensitive copy-sheet as defined in claim 2 in which theacid-progenitor means is a fiuoborate complex of a substituted-benzenediazonium radical.

4. A heat-sensitive copy-sheet as defined in claim 2 in which theacid-progenitor means is a hexafiuorophosphate complex of asubstituted-benzene diazonium radical.

5. A heat-sensitive copy-sheet as defined in claim 1 in which theacid-progenitor means includes a halogencontaining organic compound anda source of active hydrogen atoms, said compound being furthercharacterized as being reactive with aqueous alcoholic silver nitratesolution, with formation of silver halide.

6. A heat-sensitive copy-sheet as defined in claim 5 in which the sourceof active hydrogen atoms is a nontertiary amine.

7. A heat-sensitive copy-sheet as defined in claim 1 in which theinter-reactant progenitors are further characterized as being capable offorming an oxazine dye colorbody when mixed together as dilute solutionsunder acidic conditions at moderately elevated temperatures.

I 8. A heat-sensitive copy-sheet as defined in claim 1 in ReferencesCited in the file of this patent which the inter-reactant progenitors'aIC further characterized as being capable of forming an azo dyecolor-body when mixed together as dilute solutions under acidic2,663,655 m at 1953 conditions at moderately elevated temperatures. 52563556 E at 22, 1953 9. A heat-sensitive copy-sheet as defined in claim1 in 27481024 Khmkowskl et May 1955 which the inter-reactant progenitorsare further char-ac- 21838994 P et June 1958 terized as being capable offorming a Schiifs base color- 23731668 Khmlfowskl et 1959 body whenmixed together as dilute solutions under acidic 2-899334 Crevlmg et 1959conditions at moderately elevated temperatures. 10 2,940,812 D et a1June 14, 1960

1. A HEAT-SENSITIVE COPY-SHEET ADAPTED FOR MAKING A CLEAR AND SHARPREPRODUCTION OF A GRAPHIC ORIGINAL BY A DRY THERMOGRAPHIC PROCESSINVOLVING BRIEF APPLICATION OF A HEAT-PATTERN CORRESPONDING TO SAIDORIGINAL, SAID COPYSHEET BEING VISIBLY STABLE UNDER NORMAL STORAGECONDITIONS AND BEING RAPIDLY PERMANENTLY VISIBLY CHANGED ON HEATING TO ACONVERSION TEMPERATURE WITHIN THE APPROXIMATE RANGE OF 90-150*C., SAIDCOPY-SHEET INCLUDING A VISIBLY HEAT-SENSITIVE LAYER CONTAINING, ININTIMATE ASSOCIATION, (1) WEAKLY COLORED, NORMALLY SOLID, ESSENTIALLYNON-HYGROSCOPIC AND NON-VOLATILE, INTER-REACTANT PROGENITORS OFINTENSELY COLORED ORGANIC COLOR-BODIES CONTAINING DOUBLY BONDED NITROGENAS AN ESSENTIAL CHROMOPHORE, ONE OF SAID PROGENITORS BEINGACID-CONVERTIBLE TO A MODIFICATION WHICH UNDER ACIDIC CONDITIONS ISRAPIDLY VISIBLY IRREVERSIBLY INTER-REACTIVE WITH ANOTHER OF SAIDPROGENITORS, AND (2) ACID-PROGENITOR MEANS, STABLE UNDER SAID NORMALSTORAGE CONDITIONS AND LIBERATING A LEWIS ACID ON BEING MOMENTARILYHEATED AT SAID CONVERSION TEMPERATURE.